Well, I'll just assume Kento had the condom ready because he was that sure of himself and not Chia although we know she's not the example of onee-san someone should have. Just look when and why she confessed about kissing him while he was asleep. I didn't even expect her to confess that and would just keep doing it even after getting together... but it looks like she'll actually do it anyway. As much I kinda agree with the reasons Chia reverted back to black hair, I won't complain on her wearing glasses regularly now. But even of this story has so many wrong stuff about dealing with mental health problems, I'm glad it worked in the end for both of them and hopefully, Chia keeps being her mother-in-law's drinking buddy and still close to Chie and everyone is happy in the end.
Well, except for Fuuko but I think the author just forgot about her
Thanks
@SquigglesJP for the translations of this story that has no right to get me that invested for so many months. But it was a great read.